1. Field of the Invention
This invention relates to a circuit for driving an ink-jet head, particularly to a circuit for sequentially driving adjacent channels at high speed. The ink-jet head includes ink-jet channels made in continuous shape, actuators provided between the channels to pressurize ink in the channels and a circuit for driving the ink-jet head. The circuit uses diodes to protect switching devices from a breakdown due to a reverse bias voltage.
2. Description of the Related Art
A well known circuit for driving an ink-jet head includes ink-jet channels made in a continuous shape, actuators consisting of pressure-producing devices provided in correspondence with the channels and electrodes for applying voltages to the pressure-producing devices. In such an ink-jet head, voltages are applied to the electrodes of the actuators. The actuators generate pressure variations inside the channels to jet ink from the channels.
FIG. 3 is a more detailed construction of the ink-jet head. The ink-jet head 1 is provided with a piezoelectric board 2 having channel walls 11A, 11B, 11C, 11D, 11E. The channel walls 11A-11E form pressure producing devices. A part of the channel walls 11A-11E is made by a piezoelectric material formed integrally with the channel wall and polarized in an upright direction. Referring to FIG. 5, a cover plate 3 is fastened on the top of the piezoelectric board 2 through an adhesive layer 4. The ink-jet head 1 is divided into the piezoelectric board 2, cover plate 3, which serves as the upper wall of the ink chambers, and channel walls 11A-11E so as to form a number of channels (for example, 64 channels) 8A, 8B, 8C, 8D, 8E, and 8F that supply ink ejected for printing.
The channels 8A-8F are made shallow near a rear end of the piezoelectric board 2. The cover plate 3 is provided with an ink supply port 21. The ink supply port 21 is connected to a manifold 22 formed in the cover plate 3. Ink cartridges (not illustrated) connect the manifold 22 through the ink supply port 21 to supply ink to the rear ends of the channels 8A-8F.
A nozzle plate 31 is fastened by an adhesive on the front of the piezoelectric board 2 and the cover plate 3. Nozzles 32 are formed in the nozzle plate 31 in correspondence with the channels 8A-8F.
On both sides of the channel walls 11A-11E of each of the channels 8A-8F, electrodes 13 are provided to generate drive fields substantially perpendicular to the polarized direction of the respective piezoelectric materials of the channel walls 11A-11E. Conductive patterns 42 corresponding to the respective channel walls 11A-11E are formed on a board 41. The conductive patterns 42 are connected to the respective electrodes 13 using wires 43 by wire bonding.
As shown in FIG. 4, a head driver 51 for controlling ink-jet driving is connected to the conductive patterns 42. A control signal enters the head driver 51 through a line 53. A power supply voltage V is applied to the head driver 51 through a line 54, and a line 55 is provided for grounding. FIGS. 5 and 6 show the channels 8A-8F, the channel walls 11A-11E interposed between the channels 8A-8F, and electrodes 13A1, 13A2, 13B1, 13B2, 13C1, 13C2, 13D1, 13D2, 13E1, 13E2, 13F1, and 13F2 provided in correspondence with the channels 8A-8F, respectively. FIG. 5 is a longitudinal sectional view of the ink-jet head 1 showing a state with no voltage applied to the electrodes 13A1-13F2. FIG. 6 is a longitudinal sectional view of the ink-jet head 1 showing a state with a specific ON voltage applied to the electrodes 13C1 and 13C2.
Next, the ink jet motion by the ink-jet head 1 will be briefly described. Applying a specific ON voltage to the electrodes 13C1 and 13C2 and grounding the electrodes 13B2 and 13D1 will deform the channel walls 11B and 11C, as shown in FIG. 5 and FIG. 6, according to the piezoelectric perpendicular slip effect. The volume of the channel then varies to pressurize and jet the ink in the channel 8C.
FIG. 7 is a circuit diagram showing a connection of an equivalent circuit and a driving circuit for the foregoing ink-jet head. The adjacent electrodes 13A2 and 13B1 and the channel wall 11A interposed between the two adjacent electrodes 13A2 and 13B1 form an electrostatic capacitor C.sub.1 (condenser). Similarly, electrostatic capacitors C.sub.1 -C.sub.5 are formed by the adjacent electrodes 13B2 and 13C1 and the channel wall 11B, the adjacent electrodes 13C2 and 13D1 and the channel wall 11C, the adjacent electrodes 13D2 and 13E1 and the channel wall 11D, and the adjacent electrodes 13E2 and 13F1 and the channel wall 11E, respectively. Driving circuits 14A-14F are connected to electrode pairs 13Al and 13A2-13F1 and 13F2, respectively. The capacitance of electrostatic capacitors C.sub.1 -C.sub.5 are all equal to C' (farads).
Next, the electric configuration of the driving circuits and the electrodes will be explained in detail with reference to FIG. 8. In the driving circuit 14C, two switching devices, a transistor Q1 and a transistor Q2 are connected in series between a power supply V (voltage E (volts)) and a ground. One terminal of a resistor R1 (resistance R(.OMEGA.)) is connected to a connection point P1 made by the transistor Q1 and the transistor Q2, and another terminal of the resistor R1 is connected to the electrodes 13C1 and 13C2. The transistor Q1 serves to raise a voltage applied to the electrodes 13C1 and 13C2 (for charging), and the transistor Q2 serves to lower the voltage (for discharging). Base ON currents are selectively applied from a controller (not shown) to bases B1 and B2 of the transistor Q1 and the transistor Q2, respectively, to turn one transistor ON/OFF while the other transistor is OFF/ON.
In the driving circuit 14B, two switching devices, a transistor Q3 and a transistor Q4 are connected in series between the power supply V (voltage E(volts)) and the ground. One terminal of a resistor R2 (resistance R(.OMEGA.)) is connected to a connection point P2 made by the transistor Q3 and the transistor Q4, and another terminal of the resistor R2 is connected to the electrodes 13B1 and 13B2. The transistor Q3 serves to raise a voltage applied to the electrodes 13B1 and 13B2 (for charging), and the transistor Q4 serves to lower the voltage (for discharging).
In the driving circuit 14D, two switching devices, a transistor Q5 and a transistor Q6 are connected in series between the power supply V (voltage E(volts)) and the ground. One terminal of a resistor R3 (resistance R(.OMEGA.)) is connected to a connection point P3 made by the transistor Q5 and the transistor Q6, and another terminal of the resistor R3 is connected to the electrodes 13D1 and 13D2. The transistor Q5 serves to raise a voltage applied to the electrodes 13D1 and 13D2 (for charging), and the transistor Q6 serves to lower the voltage (for discharging).
When the base ON current is applied to the base B1 of the transistor Q1 from the controller (the transistor Q2 is OFF) in order to drive the ink-jet head 1 to which the electrostatic capacitors C.sub.1 -C.sub.5 are connected, a collector current Iq1 runs through the transistor Q1. The potential at the point B1 connected to the collector C1 of the transistor Q1 through the resistor R1 increases to a "H" level (E(volts)) and the potential E (volts) is applied to the electrodes 13C1 and 13C2. Accordingly, as shown in FIG. 6, channel walls 11B and 11C are deformed toward the channel 8C and pressurize ink to jet it. When the transistor Q1 is turned OFF, and the ON current is input to the base B2 of the transistor Q2, the voltage applied decreases to a "L" level (0(volts)). Then, a voltage is applied to the electrodes of the channel from which ink is to be jetted next.
When the adjacent channels 8C and 8D, for example, which share the channel wall 11C therebetween, are sequentially driven, until the walls of the first channel return to their straightened state, the walls of the second channel cannot deform. Therefore, the ink-jet interval depends on a cycle time in which the channel wall is mechanically deformed, straightened, and deformed.